Rare earth ions have been utilized in insulating crystals as the gain media for laser operations. These ions have also been incorporated into III-V semiconductor materials and into silicon in order to provide gain media in prior art laser systems. In these systems photo luminescense bands arising from the intra-center 4f--4f transitions between crystal field split spin-orbit levels of the tri-valent rare earth ions were observed at low temperatures, typically less than or equal to 77K. The wavelength of the observed emissions were found to not depend on the bandgap energy of the host semiconductor material, but rather to depend on the particular rare earth ion which was used as the dopant.
In all of the prior art systems the rare earth ions were always chosen to have a main emission wavelength which was longer than the bandgap emission wavelength of the host semiconductor material. Some of the combinations which have been studied in the prior art are as follows: erbium in GaAs, GaP, InP and Si; Neodymium, Samarium and Europium in GaP; and ytterbium in InP, GaP, and GaAs. These prior art systems can be depicted by the electron energy diagram shown in FIG. 2. FIG. 2 is an electron energy vs density of states diagram for a typical rare-earth-doped semiconductor material of the prior art where a rare earth element has been used as the dopant. As shown in FIG. 2, the energy transition (203-204) for the rare earth ion is smaller than the energy difference (205-206) for the bandgap of the semiconductor material. Accordingly, the emission wavelength for the rare earth transition is longer than the wavelength corresponding to the bandgap of the semiconductor material. As pointed out in U.S. Pat. No. 4,193,044 to C. A. Morrison et al., dated Mar. 11, 1980, the host semiconductor material in a rare earth semiconductor laser was required to have "a bandgap wide enough to be transparent to light emitted by the lasing ions".
Because the rare earth energy levels lie within the semiconductor bandgap, the electron transitions between the 4f--4f levels involves electrons which relax from the conduction band to the upper level of the rare earth ion, followed by a radiative transition 201 to the lower level of the rare earth ion, and finally by a relaxation to the valence band of the semiconductor material. Such a multi-process transition is in general less probable than the direct band-to-band transition designated as 202 in FIG. 2. As a result, the luminesecence efficiency of the rare earth transitions is extremely low and hence these transitions result in poor optical gain as indicated in FIG. 3. Accordingly, no lasing action at the rare-earth ion transition wavelength has been actually achieved in such prior art systems. As further indicated in FIG. 3, the gain for the rare earth transition and the gain for the band-to-band transitions are at distinctly different wavelengths.
Alternatively, the higher energy emission resulting from the band-to-band transition 202 of the host semiconductor material can optically pump electrons from the low level of the rare earth ion to energies higher than the upper level. Relaxation of the electrons from these higher energy levels to the upper level of the rare earth ion results in radiative transitions 207 from upper level to the lower level thereby emitting a photon at the longer wavelength. This process of pumping at the higher energy is still inefficient when compared with the direct band-to-band transition because it is a non-resonant pumping process. In both cases which result in emission from transitions between the rare earth levels, the pumping of the rare earth transition is of a non-resonant type. The quantum efficiency, i.e., conversion efficiency of input pumping power to output optical power, achieved in all of these prior art systems was in the order of 10.sup.-4.